1. Field of the Invention
This invention relates to contactless angular displacement transducers which utilize capacitive coupling to produce an electrical output signal. The phase angle of the electrical output signal is proportional to the angular displacement of a shaft.
2. Description of the Prior Art
Contactless angular displacement transducers utilizing capacitive coupling are well known in the prior art. Capacitive transducers can be grouped into three broad categories according to the manner in which the angular displacement information is provided. The first category includes capacitive transducers that produce an electrical output signal having a magnitude related to the angular displacement of a shaft. The second category includes capacitive transducers that directly produce a digital electrical output signal such that each unique combination of zeros and ones represents a position of a shaft. The third category includes those capacitive transducers that produce an electrical output signal having a phase angle related to the angular displacement of a shaft. The present invention belongs to this third category.
U.S. Pat. No. 3,702,467 for a Shaft Position Sensing Device and U.S. Pat. No. 3,873,916 for a Capacitive Sensor For Measuring Displacement Or Position are representative of the first category of capacitive transducers. In U.S. Pat. No. 3,702,467 a fixed plate has a plurality of equally spaced radial conductive bands disposed thereon with every other band connected at its inner extremity to a first annular conductive ring. The remaining conductive bands are connected at their outer extremities to a second annular conductive ring. A third annular conductive area is disposed at the center of the fixed plate. A rotor is mounted parallel to the fixed plate. The rotor has a plurality of equally spaced radial conductive bands connected to an annular conductive area disposed at the rotor's center. This annular conductive area combined with the annular conductive area of the fixed plate provides a constant capacitive pickup between the rotor and the fixed plate thus enabling brushless operation. Signals of opposite polarity are impressed on the two sets of conductive bands of the fixed plate. As the rotor moves relative to the fixed plate a triangular output signal is produced as a result of the alternate capacitive coupling between the conductive band of the rotor and the two conductive bands of the stator. By counting the peak values and the fractional peak values, i.e. measuring the voltage, the position of the rotor relative to the fixed plate may be determined.
In U.S. Pat. No. 3,873,916 a similar principle is used to provide an output signal which varies from a maximum value to a minimum value as a conductive pattern on the rotor moves from a position of maximum capacitive coupling to a position of minimum capacitive coupling with respect to a conductive pattern on the stator. The amplitude of the output voltage is proportional to the relative position of the two patterns, and the number of maximum and minimum values passed through is equal to the number of pattern divisions passed through. By counting the minimum or maximum values and by measurement of the amplitude of the output voltage the movement of the rotor relative to the stator may be determined.
Patents representative of the second category of contactless capacitive angular displacement transducers are U.S. Pat. No. 3,238,523 for a Capacitive Encoder and U.S. Pat. No. 3,766,544 for an Analog-To-Digital Converter Employing Electrostatic Signal Coupling Apparatus. In U.S. Pat. No. 3,238,523 a rotor is provided with a conductive pattern composed of several annular tracks, each track being comprised of alternating conductive and nonconductive areas. A stator is provided with a pattern composed of several pairs of annular tracks, each track being comprised of alternating conductive and nonconductive areas. As the rotor rotates closely adjacent to but slightly spaced from the stator the combination of any one rotor track with a pair of stator tracks produces a plurality of capacitances which vary with angular position of the rotor. By comparing the relative values of the capacitances within each pair of stator tracks a digital code is produced. The number of pairs of stator tracks is determinative of the number of digits in the code. The digital code thus represents the angular position of the rotor.
U.S. Pat. No. 3,766,544 discloses an analog-to-digital converter employing electrostatic signal coupling apparatus. The electrostatic signal coupling apparatus is an electrostatic encoder comprised of a stationary excitation element and a rotor vane. The stationary excitation element is divided into a plurality of excitation segments, with each segment corresponding to one of the shaft positions to be indicated. A set of excitation signals of different phases is impressed on a group of excitation segments. When an excitation signal is supplied to a segment that is under the rotor vane that signal is coupled to an output sensor due to the rotor vane. By sequentially impressing sets of excitation signals on groups of excitation segments a unique set of signals representing a shaft position is coupled to the output sensor. The set of coupled signals is translated to a set of zeros and ones to provide a digital output signal.
The third category of capacitance angular displacement transducers produces an electrical output signal having a phase angle that is related to angular displacement. An example is U.S. Pat. No. 4,007,454 for an Apparatus For Remotely Determining The Angular Orientation, Speed, And/Or Direction Of Rotation Of Objects. A rotating electric field is generated in the vicinity of a rotating member, i.e. a meter hand. The electric field has an axis of rotation coincident with the axis of rotation of the meter hand. An electric field sensing device is positioned at the rotational axis of the electric field for detecting variations of the electric field. Since the permeability of space is the same on both sides of the sensing device, the sensing device reads zero because the two halves of the rotating electric field cancel each other. However, when the electric field is coincident with the meter hand the permeability of the meter hand provides a path for the lines of flux for one-half of the electric field. This imbalance in the electric field causes a voltage signal to be picked up by the sensing device. By comparing the phase of the voltage signal picked up by the sensor to a reference signal the angular orientation of the meter hand may be determined.
The previous discussion is intended to illustrate the diversity in the prior art. Although all are examples of capacitive angular displacement transducers, different hardware and physical configurations are used to implement differing theories of operation. Accordingly, the prior art belonging to the third category is most appropriate and will therefore be discussed in further detail.
Another patent belonging to the third category of angular displacement transducers is U.S. Pat. No. 3,845,377 for a Rotation Angle Transducer. A contactless angle transducer is disclosed comprising a fixed stator plate having a plurality of electrically isolated sector members, a parallel confronting plate spaced apart from the stator plate, and a sector rotor plate situated between the stator and confronting plate and rotatably coupled to the rotatable object. AC excitation voltages having a triangular waveform and a fixed phase relationship are impressed on the sector members of the fixed stator plate. The rotor plate is used to couple the excitation voltages from the fixed stator plate to the confronting plate. Since the location of the rotor plate is a function of angular position of the rotatable object the voltage coupled to the fixed plate is also a function of angular position of the rotatable object. By comparing the phase angle of the coupled voltage to the phase angle of a reference voltage the angular position of the rotatable object can be determined.
Prior art capacitive angular displacement transducers which are used to remotely read a meter, such as a watthour meter, depend on extremely small values of capacitive coupling. Stray capacitances are not negligible compared to the small values of capacitance found in typical prior art transducers and may lead to errors. Similarly, since the capacitance is small the voltage signal coupled by the capacitance is also small making the prior art transducers very susceptible to errors due to noise or the induction of unwanted voltages. Another source of error in prior art transducers is due to the misalignment of the stator and rotor plates. This misalignment can be either a lack of concentricity between the stator and rotor plates or the stator and rotor plates may be out of parallel. The present invention is a new and useful capacitive angular displacement transducer which compensates for these errors. Other benefits will become apparent from the description of the preferred embodiment.